This present invention relates to an output arrangement for electron beam tubes, and to a travelling wave tube incorporating an output arrangement.
Electron beam tubes are used for the amplification of RF signals and are typically linear beam devices. There are various types of linear electron beam tube known to those skilled in the art, examples of which include the Travelling Wave Tube (TWT), the klystron and the Inductive Output Tube (IOT). Linear electron beam tubes incorporate an electron gun for the generation of an electron beam of an appropriate power. The electron gun includes a cathode heated to a high temperature so that the application of an electric field between the cathode and an anode results in the emission of electrons. Typically, the anode is held at ground potential and the cathode at a large negative potential of the order of tens of kilovolts.
Electron beam tubes used as amplifiers broadly comprise three sections. An electron gun generates an electron beam, which is modulated by application of an input signal. The electron beam then passes into a second section known as the interaction region, which is surrounded by a cavity arrangement including an output cavity arrangement from which the amplified signal is extracted. The third stage is a collector, which collects the spent electron beam.
In an inductive output tube (IOT) a grid is placed close to and in front of the cathode, and the RF signal to be amplified is applied between the cathode and the grid so that the electron beam generated in the gun is density modulated. The density modulated electron beam is directed through an RF interaction region, which includes one or more resonant cavities, including an output cavity arrangement. The beam may be focused by a magnetic means to ensure that it passes through the RF region and delivers power at an output section within the Interaction region where the amplified RF signal is extracted. After passing through the output section, the beam enters the collector where it is collected and the remaining power is dissipated. The amount of power which needs to be dissipated depends upon the efficiency of the linear beam tube, this being the difference between the power of the beam generated at the electron gun region and the RF power extracted in the output coupling of the RF region.
In a klystron the input signal velocity modulates an electron beam, which then enters a drift space in which electrons that have been speeded up catch up with electrons that have been slowed down. The bunches are thus formed in the drift space, rather than in the gun region itself, as in an IOT which density modulates the beam.
A Travelling Wave Tube (TWT) can be thought of as a modified type of klystron. In a TWT, a velocity-modulated beam interacts with an RF circuit known as a slow wave structure, typically either a helix or a series of cavities coupled to one another, to produce amplification at microwave frequencies. In the cavity type, the resonant cavities are coupled together with a transmission line. The electron beam is velocity modulated by an RF input signal at the first resonant cavity, and induces RF voltages in each subsequent cavity. If the spacing of the cavities is correctly adjusted, the voltages at each cavity induced by the modulated beam are in phase and travel along the transmission line to the output, with an additive effect, so that the output power is much greater than the power input.
The helix type TWT differs from other electron tubes in that it does not use RF cavities, but uses a conductive helix along which the RF wave travels. The RF energy travels along the helix wire at the velocity of light. However, because of the helical path, the energy progresses along the axial length of the tube at a considerably lower axial velocity, and hence the name “slow wave” circuit. The purpose of the slow wave structure, as in any electron beam RF interaction circuit, is to transfer energy from the electron beam to the RF signal for output. This occurs by interaction between the axial component of the electric field wave travelling down the centre of the helix and the electron beam moving along the axis of the helix at the same time. The electrons are continually slowed down as their energy transferred to the wave along the helix.
The two known types of TWT are shown in FIGS. 1 and 2. First, in FIG. 1, a TWT comprises an electron gun 14, an interaction region or circuit 18 and a collector 16. In this type of TWT, the interaction circuit comprises a series of cavities 20 coupled by a transmission line. An input 10 feeds an RF signal into the first cavity and an output 12 extracts the amplified RF signal from the TWT. Second, in FIG. 2, a TWT of the helix type comprises an electron gun 14, an interaction circuit 18 and a collector 16 as before. The interaction circuit comprises a conductive helix 4 along which the RF signal travels from an input coaxial line 7 to an output coaxial line 8. As the cavity comprises a vacuum enclosed by envelope 22, insulative rings 7A, 8A between the inner and outer conductors of the input and output coaxial lines provide a vacuum seal. An example of the helix type TWT is known from U.S. Pat. No. 4,682,076. Other known TWTs are the ring bar TWT and ring loop TWT.
With electron beam tubes that have resonant cavities, as described above, the coupling of output power is typically by an inductive loop. Where more than one output is required, more than one inductive loop may be used. However, with TWTs such as the helix type, we have appreciated that there are difficulties in providing more than one output coupling due to impedance constraints.
We have appreciated the need to improve output arrangements of electron beam tubes. In particular, we have appreciated the need to provide division of output power from devices such as TWTs in particular the helix type.